The present invention relates to a method for continuous casting of steel, particularly to a method for estimating and controlling flow pattern of molten steel in continuous casting and apparatus therefor.
Continuous casting of steel is carried out by injecting a molten steel at high speed into a mold via an immersion nozzle. The injected flow induces a molten steel flow in the mold, which molten steel flow gives significant influence on the surface and internal characteristics of produced slab. For example, when the surface flow speed of the melt surface in the mold, (hereinafter referred to simply as xe2x80x9cmeniscusxe2x80x9d), is excessively high, or when vertical eddies are generated in the meniscus, mold powder is trapped into the molten steel. In addition, it is known that the floatation of deoxidized products such as Al2O3 in the molten steel depends on the flow of molten steel. The mold powder and the deoxidized products which are trapped into the slab induce defects caused from the non-metallic inclusions on products.
Flow of molten steel in a mold varies during casting depending on the adhesion of Al2O3 to inside surface of the immersion nozzle, the erosion of the immersion nozzle, the opening of sliding nozzle, and other variables, even under the same casting condition. The phenomenon is an important issue for improving the quality of slab. To this point, there are many proposed methods to detect the flow of molten steel, to control the intensity and direction of the magnetic field to be applied based on the detected state of the molten steel flow, thus to control the flow of molten steel in the mold.
For example, Japanese Unexamined Patent Publication No. 62-252650, (hereinafter referred to simply as xe2x80x9cthe Prior Art 1xe2x80x9d), discloses a method for controlling flow of molten steel. According to the Prior Art 1, thermocouples are buried in a copper plate on shorter side of a mold to detect the difference in molten steel level between the right side and the left side to the immersion nozzle, and the direction of agitation and the thrust of agitation of the magnetic agitator are controlled to zero the level difference.
Japanese Unexamined Patent Publication No. 3-275256, (hereinafter referred to simply as xe2x80x9cthe Prior Art 2xe2x80x9d), discloses a method for controlling deflected flow of molten steel. According to the Prior Art 2, thermocouples are buried in a copper plate on longer side of a mold to measure the temperature distribution on the copper plate on longer side of the mold, and the generation of deflected flow of molten steel is detected on the basis of the temperature distribution at the right half width and the left half width of the mold, thus controlling separately the current being applied to each of the two magnetic brakes of DC magnet type, located on the rear face of longer side of the mold, responding to the detected direction and magnitude of the generated deflected flow of molten steel.
Japanese Unexamined Patent Publication No. 4-284956, (hereinafter referred to simply as xe2x80x9cthe Prior Art 3xe2x80x9d), discloses a method for controlling the speed of injection flow from an immersion nozzle in a magnetic agitator. According to the Prior Art 3, two non-contact distance meters are located above the meniscus between the immersion nozzle and the short side of the mold to measure the variations of melt level at the meniscus, and the propagation speed of the surface waves is derived from a mutual correlation function of these two measured values, thus controlling the injection flow speed from the immersion nozzle so as the propagation speed not to exceed a specified value.
The Prior Art 1 and the Prior Art 2 detect the flow of molten steel based on the temperature distribution on the mold copper plate, and control the flow on the basis of the detected molten steel flow. The variations in the temperature distribution on the mold copper plate are generated not solely caused from the variations of the flow state of molten steel, and they are generated also by the variations of the state of contact between the mold and the solidified shell, by the variations of inflow state of the mold powder, and other variables. Since there occur variations of temperature distribution on the mold copper plate owing to variables other than the flow of molten steel, the Prior Art 1 and the Prior Art 2 that detect the flow of molten steel from solely the temperature distribution on the mold copper plate cannot detect precisely the flow of molten steel.
Although no detail description is given here, investigations carried by the inventors of the present invention confirmed that, for reducing the amount of mold powder and of deoxidized products, solely the prevention of deflected flow in the mold to establish a flow symmetrical in right half width and left half width is not sufficient, and that an optimum flow pattern exists among several flows symmetrical in right half width and left half width.
The Prior Art 3 is an effective means of method for flow control. The Prior Art 3, however, controls only the flow speed of molten steel at meniscus, and is insufficient to detect the flow pattern of molten steel in the mold. In addition, both the Prior Art 1 and the Prior Art 2 cannot detect the flow pattern.
It is an object of the present invention to improve and stabilize the quality of slab manufactured by continuous casting, in particular to improve and stabilize the quality thereof through the prevention of dragging the mold powder, which is induced from a flow pattern of molten steel in the mold, thus assuring feed of good slab to succeeding stages.
In this regard, the present invention provides a method for controlling flow pattern of molten steel to maintain an optimum flow pattern in continuous casting, and further provides a temperature measurement device for mold copper plate to accurately estimate the flow state of molten steel, and a method for estimating the flow state of molten steel in the mold using the temperature measurement device.
To achieve the object, firstly, the present invention provides a method for estimating flow pattern of molten steel in continuous casting, which comprises the steps of:
continuously casting a molten steel injected into a mold through an immersion nozzle;
measuring temperatures of a copper plate in width direction thereof on longer side of the mold at plurality of points using a temperature measurement device; and
estimating a flow pattern of the molten steel in the mold based on the distribution of the copper plate temperatures at individual measurement points.
The method for estimating the flow pattern of molten steel preferably further comprises a step of applying a magnetic field to the molten steel that was injected into the mold so as the detected flow pattern to establish a specified pattern. The magnetic field applied is preferably a moving magnetic field that moves in the horizontal direction.
Furthermore, the method for estimating the flow pattern of molten steel preferably further comprises the steps of:
determining a heat flux being transferred from the molten steel in the mold to a cooling water for the mold copper plate using the mold copper plate temperatures measured by the temperature measurement device, thickness of the mold copper plate, distance between the surface of the mold copper plate on the molten steel side and the tip of a temperature measurement element, temperature of the cooling water for the mold copper plate, thickness of a solidified shell, thickness of a mold powder layer, and temperature of the molten steel in the mold;
deriving a convection heat transfer coefficient, corresponding to the heat flux, between the molten steel and the solidified shell; and
determining flow speed of the molten steel along the solidified shell based on thus derived convection heat transfer coefficient.
The method for estimating the flow pattern may further comprise the step of correcting the temperatures of copper plate on longer side of the mold.
The step of correcting the temperatures of copper plate comprises the steps of:
measuring the surface shape of the solidified shell in the slab-width direction below the lower end of the mold;
estimating the heat transfer resistance between the copper plate on longer side of the mold and the solidified shell based on thus measured surface shape; and
correcting the temperature of copper plate on longer side of the mold at every measurement point based on the estimated heat transfer resistance.
The temperature measurement device for determining the temperatures of mold copper plate applied to the method for estimating the flow pattern preferably comprises plurality of temperature measurement elements which are buried in rear face of the mold copper plate for continuous casting. The temperature measurement elements are preferably located in a distance range of from 10 to 135 mm from the level of molten steel in the mold to the direction of slab-drawing. The distance between the surface of the mold copper plate on the molten steel side and the tip of the temperature measurement element is preferably 16 mm or less, while keeping not more than 200 mm of intervals of the temperature measurement elements in the mold width direction and allotting thereof over a range corresponding to the whole width of the slab.
The step of estimating the flow pattern is preferably either one step selected from the group given below.
(A) Based on the variations of temperature of copper plate on longer side of the mold with time, the distribution of measurement points where the temperature of copper plate on longer side of the mold increases is determined. Then, based on thus determined distribution of the measurement points of temperature increase, the flow pattern of the molten steel in the mold is estimated.
(B) Based on the variations of temperature of the copper plate on longer side of the mold with time, the distribution of measurement points where the temperature of copper plate on longer side of the mold decreases is determined. Then, based on thus determined distribution of the measurement points of temperature decrease, the flow pattern of the molten steel in the mold is estimated.
(C) Based on the variations of temperature of the copper plate on longer side of the mold with time, the distribution of measurement points where the temperature of copper plate on longer side of the mold increases and decreases, respectively, is determined. Then, based on thus determined respective distributions of the measurement points of temperature increase or decrease, the flow pattern of the molten steel in the mold is estimated.
(D) Based on the number and positions of the peaks of the temperatures of mold copper plate in mold width direction, the flow pattern of molten steel in the mold is estimated.
(E) The deflected flow of the molten steel in the mold is estimated by comparing maximum value and the position of the maximum value of the temperatures of mold copper plate at right half width with maximum value and the position of the maximum value of the temperatures of mold copper plate at left half width of the mold to the center position thereof based on the measured temperatures.
Secondly, the present invention provides a temperature measurement device for mold copper plate, which comprises:
plurality of temperature measurement elements buried in tear face of a mold copper plate for continuous casting;
the temperature measurement elements being located in a distance range of from 10 to 135 mm from the level of molten steel in the mold to the direction of slab-drawing, and the distance between the surface of the mold copper plate on the molten steel side and the tip of the temperature measurement element being 16 mm or less, while keeping not more than 200 mm of intervals of the temperature measurement elements in the mold width direction and allotting thereof over a range corresponding to the whole width of the slab.
In the temperature measurement device, the temperature measurement element is preferably placed passing through a pipe which is isolated from a cooling water in a water box, and a seal packing is preferably applied around the place where the temperature measurement element is placed.
Thirdly, the present invention provides a method for judging surface defect on an slab obtained by continuous casting, which comprises the steps of:
locating plurality of temperature measurement elements in a distance range of from 10 to 135 mm from the position of meniscus in the mold to the direction of slab-drawing along the width direction of rear face of the mold copper plate;
measuring the distribution of temperatures of the mold copper plate in width direction thereof; and
judging the surface defect on the slab on the basis of the distribution of temperatures in the mold width direction.
The judgment of the defect is carried out either one selected from the group given below.
(A) The surface defect of slab is judged on the basis of the maximum value in the temperature distribution in the mold width direction.
(B) The surface defect of slab is judged on the basis of the minimum value in the temperature distribution in the mold width direction.
(C) The surface defect of slab is judged on the basis of the average value in the temperature distribution in the mold width direction.
(D) The surface defect of slab is judged on the basis of the difference between the average value of the temperature distribution in the mold width direction and the average value of a typical temperature distribution in the mold width direction at the slab-drawing speed.
(E) The surface defect of slab is judged on the basis of the larger value of, centering the immersion nozzle located at center of the mold, the difference between the maximum value and the minimum value in the temperature distribution at left half width of the mold and the difference between the maximum value and the minimum value in the temperature distribution at right half width of the mold.
(F) The surface defect of slab is judged on the basis of the absolute value, centering the immersion nozzle located at center of the mold, between the maximum value in the temperature distribution at left half width of the mold and the maximum value in the temperature distribution at right half width of the mold.
(G) The surface defect of slab is judged on the basis of the maximum value of temperature variations per unit time among the temperatures measured by every temperature measurement element.
Fourthly, the present invention provides a method for detecting the flow of molten steel in continuous casting process, which comprises the steps of:
locating plurality of temperature measurement elements orthogonally to the direction of slab-drawing at rear face of the mold copper plate for continuous casting;
measuring mold copper plate temperatures using these plurality of temperature measuring elements;
applying low pass filter treatment to each of thus measured mold copper temperatures assuming a range of cut-off space frequency of larger than [2/(mold width W)] and less than 0.01, in which the space frequency f of the molten steel flow is defined by f=1/L, where L designates varying wave length (mm); and
estimating the state of flow of molten steel in the mold on the basis of the temperature distribution of the mold copper plate, which temperature distribution was treated by the low pass filter.
The method for detecting the flow of molten steel preferably adjusts the distance between adjacent temperature measurement elements to a range of from more than 44.3/3 mm and less than [0.443xc3x97(mold width W)/6] mm.
Furthermore, the method for detecting the flow of molten steel preferably applies low pass filter treatment using a data series which is extended by doubling back the acquired data at each of both edges of the mold width.
Fifthly, the present invention provides a method for detecting the flow of molten steel in continuous casting, which comprises the steps of:
locating plurality of temperature measurement elements orthogonally to the direction of slab-drawing while keeping the distance between adjacent temperature measurement elements to a range of from 44.3/3 mm to [0.443xc3x97(mold width W)/6] mm;
measuring temperatures of a mold copper plate using thus located temperature measurement elements;
deriving a spatial movement average of thus measured mold copper plate temperatures; and
estimating a state of molten steel flow in the mold based on the temperature distribution of the spatial movement average mold copper plate temperatures.
Sixthly, the present invention provides a method for evaluating irregularity in heat-release in the mold in continuous casting, which comprises the steps of:
locating plurality of temperature measurement elements orthogonally to the direction of slab-drawing at rear face of the mold copper plate for continuous casting;
measuring temperatures of the mold copper plate using thus located temperature measurement elements;
applying low pass filter treatment to each of thus measured mold copper temperatures; and evaluating the irregularity in heat-release in the mold on the basis of the difference between the measured mold copper plate temperature and the mold copper plate temperature that was treated by the low pass filter.
Seventhly, the present invention provides a method for detecting the flow of molten steel in continuous casting, which comprises the steps of:
locating plurality of temperature measurement elements orthogonally to the direction of slab-drawing at rear face of the mold copper plate for continuous casting;
measuring temperatures of the mold copper plate using thus located temperature measurement elements;
sampling thus measured individual mold copper plate temperatures at intervals of not more than 60 seconds; and
estimating the state of molten steel flow in the mold on the basis of the mold copper plate temperatures sampled at the intervals.
Eighthly, the present invention provides a method for controlling the molten steel flow in continuous casting, which comprises the steps of:
measuring temperature distribution in the width direction of the copper plate on longer side of the mold by locating plurality of temperature measurement elements in the width direction of and on rear face of the copper plate on longer side of the mold for continuous casting; and
adjusting one or more of the variables of the magnetic field intensity of a magnetic field generator attached to the mold, the slab-drawing speed, the immersion depth of the immersion nozzle, and the Ar gas injection rate into the immersion nozzle, so as the difference between the maximum value and the minimum value in thus determined temperature distribution to become 12xc2x0 C. or less.
In the method for controlling the molten steel flow, it is preferable that one or more of the variables of the magnetic field intensity of the magnetic field generator attached to the mold, the slab-drawing speed, the immersion depth of the immersion nozzle, and the Ar gas injection rate into the immersion nozzle, are adjusted so as the difference between the maximum value and the minimum value in the measured temperature distribution to become 12xc2x0 C. or less, and so as the temperature difference between symmetrical positions in the right half width and the left half width to the immersion nozzle in width direction of the copper plate on longer side of the mold to become 10xc2x0 C. or less.
In the method for controlling the molten steel flow, it is preferable that the intensity of magnetic field of the magnetic field generator attached to the mold is adjusted separately in the right half width and the left half width of the mold to the immersion nozzle to each other.
Ninthly, the present invention provides a method for controlling the molten steel flow in continuous casting, which comprises the steps of:
measuring temperature distribution in the width direction of the copper plate on longer side of the mold by locating plurality of temperature measurement elements in the width direction of and on rear face of the copper plate on longer side of the mold for continuous casting;
deriving molten steel flow distribution in width direction of the copper plate on longer side of the mold by determining the flow speed of the molten steel at each measurement point on the basis of thus measured temperatures;
adjusting one or more of the variables of the magnetic field intensity of the magnetic field generator attached to the mold, the slab-drawing speed, the immersion depth of the immersion nozzle, and the Ar gas injection rate into the immersion nozzle, so as the difference between the maximum value and the minimum value in the determined molten steel flow distribution to become 0.25 m/sec or less.
In the method for controlling the molten steel flow, it is preferable that one or more of the variables of the magnetic field intensity of the magnetic field generator attached to the mold, the slab-drawing speed, the immersion depth of the immersion nozzle, and the Ar gas injection rate into the immersion nozzle are adjusted so as the difference between the maximum value and the minimum value in the derived molten steel flow distribution to become 0.25 m/sec or less, and so as the difference in flow speed of molten steel between symmetrical positions in the right half width and the left half width to the immersion nozzle in width direction of the copper plate on longer side of the mold to become 0.20 m/sec or less.
In the method for controlling the molten steel flow, it is preferable that the intensity of the magnetic field generator attached to the mold is adjusted separately in the right half width and the left half width of the mold to the immersion nozzle to each other.